The present invention relates to a method of driving a thermal head used in a printing device for printing letters, images and the like.
There has been known a printing device which has a thermal head constituted by a plurality of thermal resistance elements arranged in a line, and controls the number of heating pulses to be applied to each of the thermal resistance elements, thereby controlling the density of the picture element corresponding to each of the thermal resistance elements in response to the number of the pulses.
According to such printing apparatus, the printing for one line by the thermal head is carried out as follows. The thermal resistance elements for one line are electrically separated into two blocks in the middle. First, all pulses corresponding to the densities are applied for one block, then all pulses corresponding to the densities are applied for the other block. More specifically, assuming that the numbers of pulses corresponding the lowest and the highest densities are "0" and "64", respectively, the pulses of at most "64" are applied to the thermal resistance elements of the first block to perform the printing for this first block, thereafter the printing for the second block is performed.
In the conventional method performing the printing for the second block after completion of all printing for the first block, the temperature of the thermal resistance element at the boundary portion between the first and second blocks becomes higher than that at the other portion, resulting in the inconvenience that the density of the picture element at this boundary becomes higher than that at the other portion.